[Amps] How to test a balun?

[Karl-Arne Markström]

The "classical" way to check balun performance, at least on lower 
frequencies, is to connect a balanced resistive load with the correct 
impedance and the center tap "grounded".

Using an RF voltmeter or an oscilloscope, it can be checked that the correct 
voltage relationships exist between "ground" and both ends of the load. An 
oscilloscope or vector voltmeter also gives the phase balance, which should 
be shown as 180 degrees opposite at each load end.

This becomes increasingly difficult with increasing frequency, and the RF 
voltmeter or oscilloscope input impedances usually influence the results at 
UHF frequencies.

It may be possible to measure this using very loose coupling between the 
load and the transmission port of a vector network analyser capable of 
measuring large attenuations. The main difficulty would be to avoid stray 
coupling between the "ends" of the load and the network analyser input.

A reference phase and amplitude value should be established at one "end" of 
the load, and the measurement repeated at the other. Deviations
from the reference amplitude value indicate the amplitude balance, and from 
180 degrees indicate phase balance.

There are special test sets available for measuring balanced devices such as 
baluns, see i.a. www.agilent.com/find/balanced

Also, some guidance can be found in two Mini-Circuits application notes;

http://www.minicircuits.com/pages/pdfs/howxfmerwork.pdf

and

http://www.minicircuits.com/pages/BalunApplicationNote.htm

73/

Karl-Arne
SM0AOM

----- Original Message ----- 
From: "Dr. David Kirkby" <david.kirkby at onetel.net>
To: "'AMPS'" <amps at contesting.com>
Sent: Sunday, May 25, 2008 7:47 PM
Subject: [Amps] How to test a balun?


> Does anyone know how best to test a balun, consisting of a quater wave
> section at about 1 GHz. Its quite a common design, with a tube connected
> to the braid of a coax quarter wave from the end.
>
> I'm not sure how best to test this - near field probe around the coax ??
>
> Dave
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps


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